Light emitting diodes (hereinafter, referred to as “LED”) can be used with or incorporated in various devices including a tail light for vehicles, indication lamps or backlighting devices for various display devices and mobile equipment such as cellular phones. The growth in demand for such LED devices is expected to increase in the field of headlights for vehicles, backlights for liquid crystal display devices, general lighting fixtures and the like, in the future. In particular, when an LED is used as a light source for headlights for a vehicle or a general-purpose lighting fixture, it is common for the device to emit white light. It should be noted that the color of light emitted from an LED can be determined in accordance with the magnitude of band gap of a semiconductor layer that is inherent to the semiconductor crystal that is used. Accordingly, the color of light emitted from an LED can be monochrome, such as red, green, blue, or the like. When white light is desired to be obtained from an LED having such light emission characteristics, the following methods can be considered.
One method is to employ a combination of a red LED, a green LED and a blue LED arranged in line and allow them to simultaneously emit respective color light. In this case, however, the resulting light as a whole may appear to have different color dependent on various viewing directions because of the specific directivity for LEDs. In addition, it is difficult to maintain over a long period of time the emission of white light by color mixture of light emitted from three colored LEDs. This is because the colored LEDs have respective temperature dependency or other environmental dependency or different degradation rates.
An other method is to employ a particular color LED such as a blue LED and a wavelength conversion material such as a YAG (yttrium aluminum garnet) phosphor in combination. This type of phosphor can absorb blue light and emit yellow light having a longer wavelength characteristic than that of blue light. When combining the blue LED and the phosphor, a part of the blue light emitted from the blue LED can be converted into yellow light by the phosphor and the other part of the blue light can pass through the layer of the phosphor without any substantial change (i.e., while maintaining the original color of light). The color yellow is a result of the combination of red and green. Accordingly, mixture of yellow light from the phosphor with blue light from the blue LED can produce white light. This method can achieve the production of white light with a single blue LED chip, meaning the configuration can be simple. Furthermore, the light emitting apparatus produced in accordance with this method can achieve reduced cost when compared with the first method in which RGB LEDs are arranged in line.
Japanese Patent Application Laid-Open No. Hei. 9-153645 (JP1997-153645A1) discloses a semiconductor light emitting element. The semiconductor light emitting element is produced by sequentially stacking, on a growth substrate, a buffer layer, an n-type semiconductor layer, a light emitting layer, a p-type semiconductor layer, a contact layer, and an electrode layer. The semiconductor light emitting element further includes a phosphor-containing layer formed on the electrode layer. The phosphor-containing layer can absorb UV light emitted from the light emitting layer and convert it into visible light.